This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Fiber Couplers”.
1. When considering source-to-fiber coupling efficiencies, the ________ is an important parameter than total output power.
a) Numerical aperture
b) Radiance of an optical source
c) Coupling efficiency
d) Angular power distribution
Explanation: Radiance is the optical power radiated into a unit solid angle per unit emitting surface area. Since this optical power is dependent on radiance, radiance is much important factor than optical power.
2. It is a device that distributes light from a main fiber into one or more branch fibers.
a) Optical fiber coupler
b) Optical fiber splice
c) Optical fiber connector
d) Optical isolator
Explanation: Nowadays, requirements to divide combined optical signals for applications are increasing. Optical fiber coupler is one such device that is used for dividing and combining optical signals. It is generally used in LANs, computer networks etc.
3. Optical fiber couplers are also called as ________________
c) Directional couplers
Explanation: Optical fiber couplers are passive devices. The power transfer in couplers takes place either through the fiber core cross-section by butt jointing the fibers or by using some form of imaging optics between the fibers. It distributes light from one fiber to many fibers and hence it is also called as a directional coupler.
4. How many types of multiport optical fiber couplers are available at present?
Explanation: Multiport optical fiber couplers are subdivided into three types. These are three and four port couplers, star couplers and wavelength division multiplexing (WDM) couplers. These couplers distribute light among the branch fibers with no scattering loss.
5. The optical power coupled from one fiber to another is limited by
a) Numerical apertures of fibers
b) Varying refractive index of fibers
c) Angular power distribution at source
d) Number of modes propagating in each fiber
Explanation: When two fibers are coupled to each other, the optical power is limited by number of modes propagating in each fiber. For example, when a fiber propagating with 500 modes is connected to a fiber that propagates only 400 modes, then at maximum, only 80% of power is coupled into the other fiber.
6. ________ couplers combine the different wavelength optical signal onto the fiber or separate the different wavelength optical signal output from the fiber.
Explanation: WDM coupler is abbreviated as wavelength division multiplexing coupler. It is a category of multiport optical fiber couplers. It is designed to permit a number of different peak wavelength optical signals to be transmitted in parallel on a single fiber.
7. How many fabrication techniques are used for 3 port fiber couplers?
Explanation: There are two fabrication techniques available for three port couplers. First is a lateral offset method which relies on the overlapping of the fiber end faces and the other is the semi-transparent mirror method. Using these techniques, three port couplers with both multimode and single-mode fibers can be fabricated.
8. The most common method for manufacturing couplers is ____________
a) Wavelength division multiplexing
b) Lateral offset method
c) Semitransparent mirror method
d) Fused bi-conical taper (FBT) technique
Explanation: The FBT technique is basic and simple. In this technique, the fibers are generally twisted together and then spot fused under tension such that the fused section is elongated to form a bi-conical taper structure. A three port coupler can be obtained by removing one of the input fibers.
9. Couplers insertion loss is same as that of excess loss. State whether the given statement is true or false.
Explanation: Excess loss is defined as the ratio of input power to output power. The insertion loss is defined as the loss obtained for a particular port-to-port optical path. Thus, the insertion loss and excess loss are different in nature.
10. A four-port multimode fiber FBT coupler has 50 μWoptical power launched into port 1. The measured output power at ports 2,3 and 4 are 0.003, 23.0 and 24.5 μW respectively. Determine the excess loss.
a) 0.22 dB
b) 0.33 dB
c) 0.45 dB
d) 0.12 dB
Explanation: Excess loss is a ratio of power input to power output of the fiber and it is given by Excess loss = 10log10 P1/(P3+P4)
WhereP1, P3, P4 = output power at ports 1,3 and 4 resp.
11. A four-port FBT coupler has 60μW optical power launched into port one. The output powers at ports 2, 3, 4 are 0.0025, 18, and 22 μW respectively. Find the split ratio?
Explanation: Split ratio indicates the percentage division of optical power between the outputs ports. It is given by
Split ratio =[ P3/(P3+P4)]*100%
Where P3 and P4 are output powers at ports 3 and 4 respectively.
12. How many manufacturing methods are used for producing multimode fiber star couplers?
Explanation: The manufacturing methods of star couplers are mixer-rod technique and FBT technique. In the mixer-rod method, a thin platelet of glass is employed, which mixes light from one fiber, dividing it among the outgoing fibers. FBT method involves twisting, heating and pulling of fiber.
13. Calculate the splitting loss if a 30×30 port multimode fiber star coupler has 1 mW of optical power launched into an input port.
a) 13 dB
b) 15 dB
c) 14.77 dB
d) 16.02 dB
Explanation: The splitting loss is related to the number of output ports N of a coupler. It is given by-
Splitting loss (Star coupler) = 10log10N (dB).
14. A _____________ coupler comprises a number of cascaded stages, each incorporating three or four-port FBT couplers to obtain a multiport output.
Explanation: A star coupler can be realized by constructing a ladder coupler. It consists of many cascaded stages. If a three-port coupler is used, then a ladder coupler does not form symmetrical star coupler. It is a useful device to achieve a multiport output with low insertion loss.
15. A number of three-port single-mode fiber couplers are used in the fabrication of a ladder coupler with 16 output ports. The three-port couplers each have an excess loss of 0.2 dB along with a splice loss of 0.1 dB at the interconnection of each stage. Determine the excess loss.
a) 1.9 dB
b) 1.4 dB
c) 0.9 dB
d) 1.1 dB
Explanation: The number of stages M within the ladder design is given by 2M=16. Hence M=4.
Thus, excess loss is given by-
Excess loss = (M×loss in each 3-port coupler) + (Number of splices×Loss in each stage)
Where number of splices = 3 (as the value of M is equal to 4).
Sanfoundry Global Education & Learning Series – Optical Communications.
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